Report: Pentagon Investigating Friendly Fire that Killed Anti-ISIS Fighters in Syria

Smoke rises from Islamic State group positions after an airstrike by U.S.-led coalition warplanes as Iraqi counterterrorism forces face off with Islamic State militants in the Nuaimiya neighborhood of Fallujah, Iraq, Wednesday, June 1, 2016. The U.N. children's fund has issued a stark warning to Iraqi troops and Islamic State militants to spare the children amid a battle to retake the city of Fallujah city. UNICEF says it's concerned for the safety of an estimated of 20,000 children trapped inside this city west of Baghdad as very few families have managed to flee Fallujah since the offensive started more than a week ago. (AP Photo/Khalid Mohammed)

A U.S. warplane hit forces belonging to an allied Sunni Muslim militant group that Washington had trained to fight against ISIS in an apparent case of friendly fire, the Wall Street Journal is reporting.

The Pentagon, as a result, has launched an internal investigation into the matter, the paper said.

“The coalition was subsequently informed that one of the airstrikes may have hit counter-ISIL forces and their vehicle, instead of ISIL forces. Initial reports were that four counter-ISIL personnel may have been killed in the strike,” Centcom spokesman Colonel Patrick Ryder said.

On May 27 and May 28, American forces carried out airstrikes around the city of Ma'ra, Syria.

The strikes hit Islamic State, known as ISIS or ISIL, forces, but the Pentagon-backed Mutasim Brigade said an airstrike hit its troops, leaving 10 fighters dead, according to the WSJ report.

Smoke rises from the Syrian city of Kobani, following an airstrike by the U.S.-led coalition. (AP Photo/Vadim Ghirda)

A statement issued by Centcom at the time said that, “near Mar'a, five strikes struck five separate ISIL tactical units and destroyed four ISIL fighting positions, an ISIL tactical vehicle, and two ISIL vehicles.”

Centcom confirmed that a friendly fire incident indeed took place, but said there were a smaller number of casualties than what Mutasim reported. “Initial reports were that four counter-ISIL personnel may have been killed in the strike,” Ryder said.

“An investigation into this incident has been initiated. We will continue our support to counter-ISIL forces and will apply any lessons learned derived from the investigation to improve our operations in the future,” Ryder noted.

Smoke rises after airstrikes by U.S.-led coalition warplanes as Iraqi security forces advance their positions in the southern neighborhoods of Fallujah to retake the city from Islamic State militants, Iraq, Sunday, June 12, 2016. (AP Photo/Osama Sami)

Mustafa Sejry, the head of Mutasim’s office, said the brigade felt “betrayed” by U.S. forces. “They apologized and stated that it was by mistake,” he said. “And we have no other choice than to say maybe it was [a] mistake.”

The errant airstrike appears to be the first friendly-fire event acknowledged by U.S. Central Command involving U.S. forces in Syria. Another incident took place in Iraq in December 2015, and it was publicly disclosed by Centcom at the time.